Formation Clay Chemistry

Publication Rights Reserved Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and considered for publication in one of the two SPE magazines with the paper. It is apparent that improvement in the chemistry of drilling fluids leading to improved drilling efficiency is dependent upon increased knowledge of sub-surface geology and geochemistry. Because of the significance and prevalence of sub-surface clays and shales, the study of formation clay chemistry is of fundamental importance to drilling production. This paper presents certain aspects of formation clay chemistry that open new avenues of approach in drilling fluid design. Of utmost significance in formation clay chemistry is the avidity with which hydroxyl ions react with clays. This hydroxyl reaction of clays was found to cause a radical increase in total surface area and rate of cleavage in non-swelling clays such as illite, as well as clays of the expanding lattice type. It is indicated, therefore, that it is the hydroxyl reaction of clays that is primarily responsible for the weakening of the borehole formations and "mud-making" drilling fluid tendencies. Clay base exchange reactions promoted by aqueous drilling fluids of selected cation composition are indicated to be more detrimental than beneficial when viewed from the standpoint of formation chemistry. In fact, non-reactivity of the drilling fluid with formation clays appears to be a most important drilling fluid requisite. Non-reactivity or inertness of the water phase is shown to be achieved by maintaining the absence of base exchanging cations and hydroxyl ions, and is distinct from the older ideas of inhibition or "inhibited muds". Because of chemical reactivity and mechanical effects, it is concluded that any invasion of drilling fluid media into borehole or producing formations is damaging no matter whether the liquid is aqueous or non-aqueous. In fact, it appears that advantages previously claimed for certain drilling fluids were probably due to filtration control rather than composition of the liquid phase.